Method for preparing metal particles

ABSTRACT

A method for preparing metal particles, including the steps of: depositing a metal layer on a substrate, irradiating the metal layer with a laser, a plate of a material transparent or quasi-transparent to the laser wavelength being interposed between the metal layer and the laser source.

FIELD OF THE INVENTION

The invention mainly relates to a method for preparing metal microparticles and nanoparticles by laser irradiation of a metal layer.

The present invention is especially advantageous in all fields where metal particles may be used, such as, in particular, and non limitingly, catalysis, electronics, or health.

BACKGROUND

The preparation of metal microparticles (from 10⁻⁷ to 10⁻⁴ m) and nanoparticles (<10⁻⁷ m) has been the subject of many works which have enabled to develop various methods of synthesis, especially chemical (vapor phase, liquid phase, solid or mixed medium), physicochemical (vapor-phase deposition), physical, mechanical (high-energy milling or wear).

Indeed, optimizing the synthesis conditions enables to obtain as narrow as possible a particle size distribution, but also to favor a specific particle size. Controlling these parameters is a major challenge, the properties of the particles of a same metal being capable of varying according to their size and their shape. For example, magnetic particles in the form of rods do not necessarily have the same properties as particles of the same metal but smaller, and in the form of spheres.

The most current methods for synthesizing metal particles especially comprise vapor-phase, liquid-phase, solid- or mixed-medium chemical methods.

As concerns physicochemical synthesis, methods using a thermal plasma, changes of phase such as evaporation and condensation, or physical vapor phase deposition are among the most currently used.

BRIEF DESCRIPTION OF THE INVENTION

The Applicant has developed a method enabling to physicochemically prepare metal particles, by laser irradiation of a thin metal layer. This method especially concerns laser ablation, which comprises irradiating a target of a given material with a focused laser beam.

More specifically, the present invention concerns a method for preparing metal particles, comprising:

-   -   depositing a metal layer on a substrate;     -   irradiating the metal layer with a laser;         a plate of a material transparent to the laser wavelength being         interposed between the metal layer and the laser source.

“Transparent material” or “material transparent to the laser wavelength” means a material transparent or quasi-transparent to the laser wavelength, that is, a material which, advantageously, does not absorb the incident laser energy, or only very little, generally less than 20%.

The laser used advantageously is of excimer or exciplex type. It thus mainly is an ultraviolet laser. However, in a specific embodiment, the laser may be a CO₂ laser in the infrared range.

Advantageously, the excimer laser used in the context of the present invention may be selected from the group comprising excimer lasers F₂ (157 nm), ArF (193 nm), KrF (248 nm), XeBr (282 nm), XeCl (308 nm), XeF (351 nm), CaF₂ (193 nm), KrCl (222 nm), or Cl₂ (259 nm).

The photochemical method of the invention is a photoablation method. It especially implements:

-   -   the absorption of the incident energy by the metal layer;     -   the heat propagation within the metal layer;     -   the breaking of chemical bonds within the metal layer; and     -   the desorption of the metal particles, the particles detach from         the substrate.

Due to the heat increase caused by the laser irradiation to which it is submitted, the metal layer melts and may vaporize in certain cases. Thus, the temperature at the surface of the metal layer may be greater than the metal melting point.

Further, as already mentioned, the laser irradiation enables to break the metal bonds within the metal layer. During the laser pulse, the heat propagates in the metal layer, and the plate of transparent material promotes the bursting of the metal layer. Indeed, in the absence of a plate of transparent material, the laser irradiation causes the swelling of the metal layer, the metal is lifted from the substrate and ejected in uncontrolled fashion. However, in both cases, small metal droplets may be formed before being ejected from the substrate by desorption. However, in the method according to the present invention, the droplets then are transformed into micro- or nanoparticles after cooling, and thus hardening, on the plate of transparent material. After ejection of the metal from the substrate surface, the particles are deposited back on the plate of transparent material. They can then be collected on said plate of transparent material, especially by plunging the plate of transparent material into an ultrasound bath.

Due to the phenomena implemented in the context of the method according to the present invention, and especially desorption, the nature of the obtained metal particles may be influenced by the metal/plastic interface but also by the nature and the thickness of the metal layer deposited on the substrate.

Advantageously, the substrate is made of a thermally-insulating material at least on the portion having the metal layer deposited thereon. More advantageously still, it is made of:

-   -   plastic (polyethylene naphthalate (PEN), or polyimide such as         KAPTON®);     -   glass;     -   polycarbonate;     -   polytetrafluoroethylene (PTFE, Teflon®);     -   acrylate polymers;     -   steel covered with a plastic layer having a 50-nm thickness.

The substrate being advantageously made of plastic, its melting temperature is generally lower than 300° C.

During the irradiation, the incident energy is superficially absorbed by the metal layer. The duration and the intensity of the laser pulse are thus adapted according to the substrate, the nature and the thickness of the metal layer.

Preferably, in the method according to the present invention, the metal layer is deposited on the substrate by prior art techniques and, more specifically, by physical vapor deposition (PVD), by chemical vapor deposition (CVD), or by printing.

In the context of the present invention, the metal layer which is irradiated is advantageously made of a thermally-conductive material, preferably made of:

-   -   a pure metal or a mixture of metals, advantageously a pure metal         selected from the group comprising gold, copper, nickel,         palladium, and silver;     -   an alloy;     -   superposed metal layers; or     -   a metal oxide, advantageously AgO, ZnO, or ITO, and mixtures         thereof. ITO (Indium Tin Oxide) is a mixture of indium oxide         (In₂O₃) and of in oxide (SnO₂).

In a specific embodiment, the metal layer may be made of:

-   -   a metal selected from the group comprising gold, nickel,         palladium, and silver; or of     -   a metal oxide, advantageously AgO, ZnO, or ITO.

Generally, the metal layer has, before irradiation, a thickness advantageously ranging between 10 and 200 nm, and more specifically in the case of a laser having a wavelength of 248 nm or of 308 nm.

In the method according to the invention, the metal layer is covered with a plate of a transparent material, advantageously made of glass, of quartz, of borosilicate, or plastic such as PEN.

In a preferred embodiment, the plate of transparent material is made of quartz. In a more specific case, the surface of the plate of transparent material opposite to the metal layer may be covered with an anti-adherent coating, advantageously a fluorinated coating. Thus, the adherence between the plate of transparent material and the substrate during the irradiation is limited.

In a specific embodiment, the non-adherent surface of the plate of transparent material may comprise fluorinated SAMs (self-assembling monolayers), for example, fluorotris[3-(trifluoromethyl)phenyl]silane grafted by hydrogen plasma, but also a fluorinated deposition obtained by plasma under SF₆ gas, or by spin coating of a fluorinated layer such as Teflon®.

The plate of transparent material is advantageously supported by the metal layer. In other words, the plate of transparent material is advantageously in contact with the metal layer. However, in another embodiment, the plate of transparent material and the metal layer may be spaced apart by a distance of at most 1 mm.

To have a good mechanical behavior, the plate of transparent material advantageously has a thickness greater than or equal to 0.5 mm. More advantageously still, it has a 1-mm thickness.

As already mentioned, the plate of transparent material is advantageously made of quartz. It thus has a relatively low ultraviolet ray absorption coefficient, unlike a conventional glass plate.

In the method according to the present invention, the metal layer is irradiated by means of a laser having a fluence advantageously ranging between 10 and 1,000 mJ/cm², and more advantageously between 10 and 200 mJ/cm². The laser fluence corresponds to the power flux density. According to the power or the fluence, but also according to the irradiation time, particles having a dimension capable of varying from 10 to 1,000 nm can thus be prepared, advantageously from 10 to 500 nm.

Further, the duration of the laser firings generally is on the order of 20 ns. It is however possible to multiply laser firings in a same area to reach the fluence implemented in the method according to the present invention. Generally, when the thickness of the metal layer is smaller than 200 nm, a single excimer laser firing may be sufficient.

Generally, the particles obtained according to the method of the present invention may be in spherical form or in the form of sheets or of plates. Accordingly, “particle dimension” mainly means the particle diameter or the dimension in the plane of the sheets or of the plates forming the metal particles.

Indeed, generally, three types of metal particle shapes may be obtained, especially for a laser having a 308-nm wavelength:

-   -   when the laser fluence is greater than or equal to 90 mJ/cm²         and/or when the thickness of the metal layer is smaller than or         equal to 60 nm, droplets are formed. After deposition of the         droplets on the plate of transparent material and hardening,         generally spherical particles are obtained, with a dimension         ranging between 10 and 60 nm;     -   when the laser fluence ranges between 50 and 90 mJ/cm² and when         the thickness of the metal layer ranges between 10 and 60 nm,         metal particles in the form of sheets are obtained, their         dimension in the plane ranging between 100 and 500 nm.

In this case, the thickness of the obtained sheets is substantially equal to that of the metal layer;

-   -   when the laser fluence is smaller than 50 mJ/cm² and/or the         thickness of the metal layer is greater than 60 nm, metal         particles in the form of plates are obtained, their dimension in         the plane being capable of being greater than 1 mm. The plate         thickness is substantially equal to that of the metal layer.

The particle shape also depends on the spacing between the plate of transparent material and the metal layer. The smaller this spacing, the more the particles have a spherical shape. Further, and as already mentioned, the shape also depends on the thickness of the metal layer.

It will be within the abilities of those skilled in the art to adjust all the parameters according to the nature of the metal layer and also according to the size and the shape of the particles which are desired to be prepared. Indeed, the values given hereabove for a laser having a 308-nm wavelength may be lower in the case of a laser having a lower wavelength.

Advantageously, the particles obtained by implementing the method according to the present invention have a spherical shape.

In a specific embodiment, when the laser fluence is lower than 70 mJ/cm², sheets or plates having an average 500-nm dimension in the plane are obtained.

In another specific embodiment, when the laser fluence is greater than 100 mJ/cm², for a metal layer having a 50-nm thickness, spherical particles having an average 50-nm dimension are obtained.

In a specific embodiment of the present invention, before irradiation, the substrate covered with the metal layer and the plate of transparent material are immersed in a liquid. Advantageously, the liquid having the substrate covered with the metal layer and the plate of transparent material immersed therein has a heat conductivity greater than that of air to absorb energy and/or an optical index different from that of the plate of transparent material to focus or defocus the incident beam.

This thus enables not only to modify the incident energy of the laser, but also to control the particle shape and dimension.

DESCRIPTION OF THE DRAWINGS

The invention and the resulting advantages will better appear from the following drawings and non-limiting examples.

FIG. 1 illustrates prior art and shows the irradiation of a metal layer in the absence of a plate of transparent material, thus highlighting the swelling of the metal layer.

FIG. 2 illustrates the irradiation of a metal layer by means of a laser, according to the method of the invention, where a plate of transparent material is deposited on the metal layer.

EMBODIMENT OF THE INVENTION

A gold metal layer having a 30-nm thickness is deposited by vacuum evaporation on a PEN (polyethylene naphthalate) plastic substrate having a 125-μm thickness.

A quartz glass plate having a 1-mm thickness is then deposited on the metal-plastic stack thus formed, which is insolated by a UV excimer laser (308 nm). The total irradiation time is 20 ns.

The quartz glass plate prevents the swelling of the metal layer, thus causing the breaking of the bonds and the ejection of thin metal droplets.

After bursting of the metal layer, the ejected gold particles are recovered on the quartz plate. Such gold particles have an average 30-nm diameter.

As illustrated in FIG. 1, in the absence of plate of transparent material (prior art), metal layer 2 deforms under the effect of the heat generated by laser irradiation 4. Indeed, since metal layer 2 and substrate 1 have different expansion coefficients, a bilayer effect 5 causes the swelling of the metal layer. In such a photoablation process, the swelling of the metal layer goes along with the ejection of the metal present in the irradiated area.

In the context of the present invention (FIG. 2), plate 3 of transparent material prevents the swelling of metal layer 2 while promoting its bursting. During laser irradiation 4, the metal is torn from substrate 1 and ejected in the form of small droplets. The plate of transparent material enables to collect the metal droplets which, after cooling, form micro- or nanoparticles. 

1. A method for preparing metal particles, comprising the steps of: depositing a metal layer on a substrate; irradiating the metal layer with a laser; a plate of a material transparent or quasi-transparent to the laser wavelength being interposed between the metal layer and the laser source.
 2. The method for preparing metal particles of claim 1, wherein the laser is an excimer laser.
 3. The method for preparing metal particles of claim 1, wherein the plate of transparent material is in contact with the metal layer.
 4. The method for preparing metal particles of claim 1, wherein the plate of transparent material and the metal layer are spaced apart by a distance of at most 1 mm.
 5. The method for preparing metal particles of claim 1, wherein the plate of transparent material is made of glass, of quartz, of borosilicate, or of plastic.
 6. The method for preparing metal particles of claim 1, wherein the laser fluence ranges between 10 and 1,000 mJ/cm².
 7. The method for preparing metal particles of claim 1, wherein the metal layer is made of: a metal selected from the group comprising gold, copper, nickel, palladium, and silver; or of a metal oxide, advantageously AgO, ZnO, or ITO.
 8. The method for preparing metal particles of claim 1, wherein the metal layer is deposited by physical vapor deposition (PVD), by chemical vapor deposition (CVD), or by printing.
 9. The method for preparing metal particles of claim 1, wherein the metal layer has a thickness ranging between 10 and 200 nm when the laser has a wavelength of 248 nm or of 308 nm.
 10. The method for preparing metal particles of claim 1, wherein the surface of the plate of transparent material opposite to the metal layer is covered with a fluorinated coating.
 11. The method for preparing metal particles of claim 1, wherein the substrate is made of a material selected from the group comprising: plastics, such as polyethylene naphthalate or polyimide; glass; polycarbonates; polytetrafluoroethylene; acrylate polymers; steel covered with a plastic layer having a 50-nm thickness.
 12. The method for preparing metal particles of claim 1, wherein the particle dimension ranges between 10 and 500 nm.
 13. The method for preparing metal particles of claim 1, wherein before irradiation, the substrate covered with the metal layer and the plate of transparent material are immersed in a liquid.
 14. The method for preparing metal particles of claim 13, wherein the liquid has a heat conductivity greater than that of air and/or an optical index different from that of the plate of transparent material. 